Department of Civil & Infrastructure Engineering

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IIT Jodhpur

Dry deposition is a major sink for tropospheric ozone; nevertheless, studies on its effects on ozone distribution are very limited over the rapidly developing South Asian region. We performed numerical simulations using the regional model WRF-Chem (Weather Research and Forecasting Model coupled with chemistry) to investigate the effect of dry deposition on surface ozone over this region by switching dry deposition ON-OFF in the model. Dry deposition of ozone is found to reduce ozone mixing ratios by up to ∼ 40% over the Indo-Gangetic Plain (IGP) and parts of western and central India. Additional enhancements (by up to ∼ 5 ppbv) in ozone are simulated when dry deposition for all gaseous species is switched off. We find a significant contrast on the effects of dry deposition from station to station as well as on the diurnal timescales over this region. The enhancements in ozone, caused by the absence of dry deposition, are lower over urban stations during nighttime, as compared to the rural and high-altitude stations. Significant enhancements in ozone levels in the absence of dry deposition over the mostly agrarian IGP underpin the importance of dry deposition particularly in the vegetated areas. The South Asian ecosystem is seen to act as an important sink of surface ozone via the dry deposition. The analyses partially fill a gap in the studies of dry deposition over the South Asian region, where this sink is anticipated to get perturbed following the changes in land use and land cover. Copyright © 2020 American Chemical Society.

World University root node

ACS Earth and Space Chemistry

Effects of Dry Deposition on Surface Ozone over South Asia Inferred from a Regional Chemical Transport Model

Delhi is one of the most polluted cities worldwide and a comprehensive understanding and deeper insight into the air pollution and its sources is of high importance. We report 5 months of highly time-resolved measurements of non-refractory PM2.5 and black carbon (BC). Additionally, source apportionment based on positive matrix factorization (PMF) of the organic aerosol (OA) fraction is presented. The highest pollution levels are observed during winter in December/January. During that time, also uniquely high chloride concentrations are measured, which are sometimes even the most dominant NR-species in the morning hours. With increasing temperature, the total PM2.5 concentration decreases steadily, whereas the chloride concentrations decrease sharply. The concentrations measured in May are roughly 6 times lower than in December/January. PMF analysis resolves two primary factors, namely hydrocarbon-like (traffic-related) OA (HOA) and solid fuel combustion OA (SFC-OA), and one or two secondary factors depending on the season. The uncertainties of the PMF analysis are assessed by combining the random a-value approach and the bootstrap resampling technique of the PMF input. The uncertainties for the resolved factors range from ±18% to ±19% for HOA, ±7% to ±19% for SFC-OA and ±6 % to ±11% for the OOAs. The average correlation of HOA with equivalent black carbon from traffic (eBCtr) is R2 = 0.40, while SFC-OA has a correlation of R2 = 0.78 with equivalent black carbon from solid fuel combustion (eBCsf). Anthracene (m/z 178) and pyrene (m/z 202) (PAHs) are mostly explained by SFC-OA and follow its diurnal trend (R2 = 0.98 and R2 = 0.97). The secondary oxygenated aerosols are dominant during daytime. The average contribution during the afternoon hours (1 pm–5 pm) is 59% to the total OA mass, with contributions up to 96% in May. In contrast, the primary sources are more important during nighttime: the mean nightly contribution (22 pm–3 am) to the total OA mass is 48%, with contributions up to 88% during some episodes in April. © 2020 Elsevier B.V.

Science of the Total Environment

Chemical characterization of PM2.5 and source apportionment of organic aerosol in New Delhi, India

Asphalt binder is a complex viscoelastic material. It implies that based on temperature and loading, its behaviour changes from a viscous material to an elastic material. Modification and ageing further add to this complexity. Over the years, a number of studies have examined all the aspects of polymer-modified binders (PMB), but there is still work needed to correlate the viscoelastic behaviour of the PMBs to their morphological behaviour. So, in the present study PMBs are prepared by modifying VG-10 base binder with the help of styrene butadiene styrene and ethylene vinyl acetate. Binders were aged using rolling thin-film oven test and pressure ageing vessel. Viscoelastic and morphological evaluation of binders was carried out by using dynamic shear rheometer and scanning electron microscopy (SEM), respectively. The test results showed that polymer modification enhances the thermorheological complexity of the base binders, while ageing does not affect it. Linear viscoelastic limits of the binders were found to be decreasing after ageing and modification. Ageing degrades the polymer network so after ageing PMBs were found to be behaving similar to base binder. The SEM results support these results. [Figure not available: see fulltext.] © 2020, Iran University of Science and Technology.

International Journal of Civil Engineering

Viscoelastic and Morphological Evaluation of Aged Polymer Modified Asphalt Binders

Analysis of observed Indian Summer Monsoon precipitation reveals more increase in extreme precipitation (in terms of its magnitude) over south India compared to north and central India during 1971–2017 (base period: 1930–1970). In the future, analysis of precipitation from the Coordinated Regional Downscaling Experiment indicates a southward shift of precipitation extremes over South Asia. For instance, the Arabian Sea, south India, Myanmar, Thailand, and Malaysia are expected to have the maximum increase (~18.5 mm/day for RCP8.5 scenario) in mean extreme precipitation (average precipitation for the days with more than 99th percentile of daily precipitation). However, north and central India and Tibetan Plateau show relatively less increase (~2.7 mm/day for RCP8.5 scenario). Analysis of air temperature at 850 mb and precipitable water (RCP4.5 and RCP8.5) indicates an intensification of Indian Ocean Dipole in future, which will enhance the monsoon throughout India. Moisture flux and convergence analysis (at 850 mb) show a future change of the direction of south-west monsoon winds towards the east over the Indian Ocean. These changes will intensify the observed contrast in extreme precipitation between south and north India, and cause more extreme precipitation events in the countries like Myanmar, Thailand, Malaysia, etc. © 2020, The Author(s).

Scientific reports

Southward shift of precipitation extremes over south Asia: Evidences from CORDEX data

Fulltext

npj Climate and Atmospheric Science

Author Correction: Future exacerbation of hot and dry summer monsoon extremes in India (npj Climate and Atmospheric Science, (2020), 3, 1, (10), 10.1038/s41612-020-0113-5)

The presence of Cr(VI) in water, even in trace concentrations, is toxic to numerous animals, plants, and microorganisms. Remediation efforts require selective removal of trace concentrations of Cr(VI) anions from the background of other competing ions present in concentrations at least 2 orders of magnitude greater than Cr(VI). The objective of this study was to evaluate the trace Cr(VI) removal capacities of commonly available anion exchangers. Batch and column studies revealed that Duolite A7 showed extraordinarily high Cr(VI) removal capacity compared to other anion exchangers. For a synthetic influent solution containing trace Cr(VI) at pH 5, Duolite A7 removed Cr(VI) for 30 000 bed volumes (BVs), while the columns having other anion exchangers reached breakthrough almost instantaneously less than 1000 BVs. Equilibrium and kinetic studies were performed on Duolite A7 at varying pH and contact time, coupled with characterization studies on the raw and exhausted ion exchanger to elucidate the mechanism behind such a significantly higher Cr(VI) removal. Based on the results, it was concluded that there was an occurrence of redox reaction within the resin in addition to ion exchange because of the unique resin composition. Duolite A7 is a weak-base anion exchange resin with a phenol formaldehyde matrix, and both secondary and tertiary amine functional groups. The redox reactions resulted in the perpetual regeneration of ion exchange sites and 30× greater Cr(VI) removal capacity of Duolite A7. This finding may pave the way to design a new class of redox-active ion exchangers for various water and wastewater treatment applications. ©

Industrial and Engineering Chemistry Research

Trace Cr(VI) Removal: Evidence of Redox-Active Ion Exchange by a Weak-Base Anion Exchanger

A tractable analytical solution for steady-state two-dimensional seepage from a trapezoidal channel in a homogeneous, isotropic porous medium of considerable depth is presented. The analysis is performed by applying the method of inversion and the Schwarz-Christoffel transformation accounting for the capillary action. Because the right half-seepage domain is a mirror image of the left half-seepage domain about the vertical axis (on either side of the channel central plane) owing to the axisymmetric channel, a solution is sought for the right half-seepage domain. The results show that an increase in channel bottom width boosts seepage flux. On the other hand, an increase in channel side slope also amplifies seepage flux. In addition, capillarity plays a subtle role in augmenting the seepage flux from a channel. The analysis suggests that the dynamic capillary rise (i.e., the vertical rise of seepage water along the side slope owing to capillary action) is always less than the static capillary rise. The analysis also presents the relation for the seepage velocity distribution along a channel perimeter. The equations of seepage line coordinates yield the seepage line initially curving downward with a significant lateral shift and eventually becoming vertical at a great depth. Particular solutions for the triangular and rectangular channels and the vertical slit, which is a channel with vertical sides and negligible top width, can be obtained from the generalized solution for the trapezoidal channel. Therefore, this study provides insight into the hydraulics of seepage from a trapezoidal channel, including its particular cases, revealing some new features. © 2020 American Society of Civil Engineers.

Journal of Hydraulic Engineering

Hydraulics of Seepage from Trapezoidal Channels

In this paper, a statistical description of turbulence in flow on a water-worked gravel-bed is presented by applying the laws of turbulence in conjunction with the double-averaging methodology. To this end, a laboratory experiment was performed, in which the gravel-bed was worked by flowing water. From the Taylor frozen-in approximation, the energy spectra of streamwise velocity fluctuation reveal the existence of the inertial subrange (lying between the energy containing and the dissipation ranges), where the turbulent kinetic energy (TKE) dissipation rate is constant. It is revealed that, in this range, Kolmogorov's 4/5-law for the spatial increments of streamwise velocity is valid, thus allowing an accurate estimation of the TKE dissipation rate. Although Monin-Yaglom's 4/3-law for the third-order mixed structure function provides a behavior similar to that of Kolmogorov's 4/5-law, the estimation of the TKE dissipation rate by Monin-Yaglom's 4/3-law is not quite accurate, owing to the departure from the isotropic turbulence at large scales. Therefore, the present study demonstrates the validity of this statistical approach (Kolmogorov's 4/5-law) to investigate the turbulence at small scales. Besides, the data plots for an anisotropy invariant map suggest that near-bed anisotropic turbulence tends to reduce to the three-dimensional isotropic turbulence with an increase in the vertical distance, indicating a relaxation of the effects of bed roughness on anisotropic turbulence toward the free surface. © 2020 Georg Thieme Verlag. All rights reserved.

Physics of Fluids

Structure functions and invariants of the anisotropic reynolds stress tensor in turbulent flows on water-worked gravel beds

Characteristics of turbulence anisotropy in flow over two-dimensional rigid dunes are analysed. The Reynolds stress anisotropy is envisaged from the perspective of the stress ellipsoid shape. The spatial evolutions of the anisotropic invariant map (AIM), anisotropic invariant function, eigenvalues of the scaled Reynolds stress tensor and eccentricities of the stress ellipsoid are investigated at various streamwise distances along the vertical. The data plots reveal that the oblate spheroid axisymmetric turbulence appears near the top of the crest, whereas the prolate spheroid axisymmetric turbulence dominates near the free surface. At the dune trough, the axisymmetric contraction to the oblate spheroid diminishes, as the vertical distance below the crest increases. At the reattachment point and one-third of the stoss-side, the oblate spheroid axisymmetric turbulence formed below the crest appears to be more contracted, as the vertical distance increases. The AIMs suggest that the turbulence anisotropy up to edge of the boundary layer follows a looping pattern. As the streamwise distance increases, the turbulence anisotropy at the edge of the boundary layer approaches the plane-strain limit up to two-thirds of the stoss-side, intersecting the plane-strain limit at the top of the crest and thereafter moving towards the oblate spheroid axisymmetric turbulence. © 2020 The Author(s).

Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences

Reynolds stress anisotropy in flow over two-dimensional rigid dunes: Reynolds stress anisotropy

The oxidative potential (OP) of ambient particulate matter (PM) is a metric commonly used to link the aerosol exposure to its adverse health effects. In this study, we report the first-ever real-time measurements of ambient PM2.5 OP based on a dithiothreitol (DTT) assay in Delhi, during a late winter season (February 2019). The chemical composition of PM was also measured using various collocated online instruments to identify the chemical components driving the PM2.5 OP. The hourly averaged OP during the entire campaign ranged from 0.49 to 3.60 nmol min-1 m-3, with an average value of 1.57 ± 0.7 nmol min-1 m-3. The secondary organic aerosols appear to be the major driver for the variation in the intrinsic OP of PM2.5. Although the average PM1 mass concentration at Delhi was 13 times the average PM2.5 mass concentration reported in Illinois, USA, in a similar study, it was not accompanied by a proportionate increase in the OP (the average volume-normalized DTT activity of PM2.5 was only 5 times that reported in Illinois). These findings reveal substantial spatial heterogeneity in the redox properties of PM and highlight the importance of determining the PM chemical composition along with its mass concentrations for predicting the overall health impacts associated with aerosol exposure. Copyright © 2020 American Chemical Society.